1) Field of the Invention
The present invention relates to an electronic timepiece and an information-processing terminal that has a secondary cell that displays how much charge is accumulated (hereinafter “accumulated condition”) of the secondary cell.
2) Description of the Related Art
Electronic timepieces (watches) that have functions of power generation such as photovoltaic power generation and mechanical power generation are known in the art. These electronic timepieces have a secondary cell for accumulating power supplied from a power supply (hereinafter “power supplier”). The power supply is, for example, a power generator such as a solar cell or a charger. The secondary cell is employed to accumulate power output from the power supplier to operate a timepiece circuit. The technology is advancing fast and secondary cells with much larger accumulation capacities are being developed.
FIG. 12 is a graph that shows a relation between accumulation capacity and voltage with time during charging of a secondary cell. The accumulation capacity of, for example, 1000 μAh means that the amount of accumulated charge that can be discharged for one hour at a constant-current discharge is 1000 μAh.
In FIG. 12 it is assumed that a solar cell supplies power to the secondary cell. The solar cell is exposed under a brightness of 40,000 luxes (like outdoors in slightly cloudy weather). As shown by a one-dot-broken-line in FIG. 12, the amount of accumulated charge increases almost in proportion to the time elapsed after the end of power supply. A cell voltage shown with a dotted line also increases almost in proportion to the charging time. Therefore, the amount of charge accumulated in the secondary cell can be generally estimated based on the cell voltage. The amount of charge accumulated in a cell is often estimated by simply detecting the cell voltage.
However, lithium-ion secondary cell that is generally used in electronic timepieces and the like causes a phenomenon called polarization. Polarization is the phenomenon wherein the cell charged with relatively large current causes only the cell voltage to elevate without accumulating the charge sufficiently and therefore disturb the relation of voltage with the amount of accumulated charge. The full-charge condition of the cell is thus wrongly detected and the charging operation is terminated. As a result, the secondary cell can not be electrically charged sufficiently. To solve the problem, such a technology is disclosed that detects a certain voltage continuously for a constant time in order to determine if it has reached a certain value of voltage (see Japanese Patent Application Laid-Open No. 1-15679).
As the secondary cells advance extremely as explained above, a similar-size cell with a larger accumulation capacity is developed and brought to the commercial stage one after another. The dotted line in FIG. 12 shows a variation in the amount of charge accumulated in the secondary cell having a larger accumulation capacity compared to the conventional cell. The conventional cell achieves 4000 μAh while the new product can achieve 25% more or 5000 μAh.
The cell voltage variation curve in the conventional art has no large variation and remains almost similar. Therefore, when the conventional detection system is employed for detecting the amount of accumulated charge, it displays a full-charge indication even before the secondary cell is fully charged. Such electronic timepiece has limitations in the maximum voltage rating of the secondary cell and the highest voltage to drive a motor for analog timepieces. Therefore, when the secondary cell voltage reaches a certain voltage (about 2.2V), an overcharge protector is activated to prevent the voltage from elevating above the certain voltage.
The cell voltage variation curve shown in FIG. 12 includes a waved part, which indicates operation of the overcharge protector. When a voltage above 2.2V is detected during intermittent detection of the cell voltage, the power supplied from the power supplier to the secondary cell is cut off to control the cell voltage so as not to greatly exceed the certain voltage. Accordingly, no charge is accumulated in the secondary cell during operation of the overcharge protector.
As can be seen from FIG. 12, there is no problem in the conventional secondary cell because the overcharge protector starts its operation when the cell is close to an almost full-charge condition. On the contrary, in the case of the secondary cell with a larger capacity, the overcharge protector reduces the supply of charge before the charge is accumulated sufficiently. Therefore, it is difficult to determine an amount of accumulated charge merely based on the cell voltage and time as in the conventional art.
This problem can be solved if the overcharge protector can operate at a higher voltage. The value of the voltage can not be changed easily, however, because it depends on a rating of the cell and a threshold voltage for driving the motor.
When a cell having a larger accumulation capacity is employed, a correct full-charge condition can not be detected and displayed due to the above background unless the voltage detection system is changed or the motor is changed to increase the drive voltage. This change causes a considerable cost-up, which results in a large problem for advancing to the commercial stage.